Sensors and Actuators B 140 (2009) 287–294 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Electrokinetic actuation of low conductivity dielectric liquids R.V. Raghavan a , J. Qin a , L.Y. Yeo a,∗ , J.R. Friend a , K. Takemura b , S. Yokota c , K. Edamura d a Micro/Nanophysics Research Laboratory, Monash University, Clayton, Victoria 3800, Australia b Department of Mechanical Engineering, Keio University, Yokohama, Japan c Precision and Intelligence Laboratory, Tokyo Institute of Technology, Japan d New Technology Management Co. Ltd., Tokyo, Japan article info Article history: Received 26 August 2008 Received in revised form 10 February 2009 Accepted 20 April 2009 Available online 3 May 2009 Keywords: Microfluidics Electrohydrodynamics Micropumping Electric pressure gradient Induced charge Polarization abstract Whilst electrohydrodynamic (EHD) flow actuation of dielectric fluids has been widely demonstrated, the fundamental mechanisms responsible for their behaviour is not well understood. By highlighting key distinguishing features of the various EHD mechanisms discussed in the literature, and proposing a more general mechanism based on Maxwell (electric) pressure gradients that arise due to induced polarization, we suggest that it is possible to identify the dominant EHD mechanisms that are responsible for an observed flow. We demonstrate this for a class of low conductivity dielectric fluids — Electro-Conjugate Fluids (ECFs) — that have recently been shown to exhibit EHD flow phenomena when subjected to non- uniform fields of low intensities. Careful inspection of the salient attributes of the flow, at least at low field strengths (<1kV/cm) — for example, the absence of a threshold voltage for the onset of flow, the quadratic scaling of the flow velocity with the applied voltage, and flow from the high to the low field region — eliminate the possibility of mechanisms based on space charge. Instead, we suggest that flow can be attributed to the existence of a Maxwell pressure gradient. This is further corroborated by good agreement between our experimental results and theoretical analysis. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The drive towards miniaturization of chemical and biological analytical systems has created the need for microfluidic actu- ation mechanisms such as micro-scale pumps, valves, reactors and separators [1]. Whilst a variety of fluid actuation schemes have been proposed, the absence of mechanical parts, lightweight construction, reliability, low power consumption and the ability to generate considerable flow velocities have led to electrohy- drodynamics (EHD) as an obvious choice for actuation in heat transfer and micropumping devices involving dielectric fluids [2]. EHD flow is induced when a dielectric fluid is subjected to a non-uniform electric field. To date, several mechanisms have been proposed in order to explain the observed EHD phenomena, namely, ion injection, conduction pumping and induction pump- ing. Whilst the first two mechanisms rely on the presence of space charge in the fluid, the latter requires charge polarization to be induced within the fluid. It seems though that these mechanisms have been proposed almost in isolation to each other, perhaps as a result of their proponents working in widely different research disciplines. Moreover, few attempts have been made to discuss ∗ Corresponding author. Tel.: +61 3 99053834; fax: +61 3 99054943. E-mail address: leslie.yeo@eng.monash.edu.au (L.Y. Yeo). the differences between these mechanisms and to suggest specific circumstances under which a mechanism dominates over others. Consequently, this has resulted in considerable confusion in the literature. In this paper we briefly discuss the underlying principles respon- sible for each of the mechanisms and summarize the key features that distinguish each of them. We believe that this will allow iden- tification of the dominant mechanism underlying a specific EHD phenomenon. We do not a priori assume any of the mechanisms dis- cussed above to be the dominant mechanism in our experiments. Instead, we allow our experimental observations and a theoreti- cal analysis to determine this through a process of elimination. By doing so, we hope to demonstrate that it is possible to iden- tify a dominant mechanism by comparing the salient features of the EHD flow with the key distinguishing attributes of the various EHD mechanisms that we will summarize in the conclusions of the paper. In addition, we also propose a more general mechanism that can generate EHD flows driven by Maxwell (electric) pressure gra- dients. We show that this is the dominant mechanism, at least when low electric field intensities are employed for a particular class of dielectric fluids known as Electro-Conjugate Fluids (ECF) that have recently been used in several EHD applications [3–5]. Our motiva- tion behind the choice of these fluids was the lack of fundamental understanding regarding their mode of actuation. From a generic viewpoint, these fluids may be treated as typical low conductivity homogenous dielectric liquids. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.04.036